The stroma of the marrow and spleen is under study. Reticular cells which form the stroma are to be isolated by trypsin digestion and differential centrifugation. The cells shall be studied in tissue culture by TEM, SEM and freeze-fracture etch. Since the stroma appears to play a role in cell sorting and migration in the splenic white pulp, isolated reticular cells from T and B cell zones in white pulp shall be mixed with T cells and B cells to determine if there is a distinctive rosette formation or influence on cell movement. SEM and TEM studies on thymus, bone marrow (after a phenylhydrazine-induced anemia and in aplastic anemia) and spleen shall be carried out. A study of the development of mast cells in tissue culture shall be undertaken by SEM, TEM and freeze-fracture etch methods. The culture is the Ginsburg-Sachs model starting with thymus cells.